[iOS]
Please contact Apple Support for information on how to re-dwonload the app.

Apple Support
https://support.apple.com/

[Android]
You can re-download apps that you have downloaded in the past free of charge by going to 'My Apps' in GooglePlay.

*Please do not forget to transfer your save data, or you will lose your progress otherwise.

You can transfer your save data by registering to 'Square Enix Bridge' before deleting the app, or transferring devices. You can find this option under by choosing 'Option' in the app menu and then selecting 'Transfer devices.

After re-downloading the app, select 'Transfer' and follow the on-screen instructions.